1. Field of the Invention
The present invention pertains to a ball end mill with a cutting insert, and particularly to the improvements for preventing the insert from being shifted during a cutting operation.
2. Prior Art
FIGS. 10 and 11 depict a conventional ball-nose end mill which includes a generally cylindrical end mill body 1 having a generally hemispherical forward end portion 1A. A main insert-receiving recess 2 and an auxiliary insert-receiving recess 3 are formed in the forward end portion 1A, diametrically opposite to each other. An indexable main cutting insert 4 and an auxiliary cutting insert 5 are mounted on the recesses 2 and 3, and secured thereto by means of clamp screws 6 and 7, respectively.
The side wall 2A of the main insert-receiving recess 2 has a concavely arcuate portion facing generally forwardly and radially outwardly from the body 1 and a pair of planar portions connected to the opposite ends of the arcuate portion. The main cutting insert 4 has side faces 4A each formed so as to be substantially mated with the above side wall 2A, and its positioning is carried out by bringing the planar portions of one of the side faces 4A into abutting contact with the planar portions of the side wall 2A.
In the above ball end mill, when an axial cutting load, as designated at L in FIG. 11, is exerted on the leading end of the main cutting insert 4, a rotational force R, to turn the main cutting insert 4 about a center O along the arcuate side wall 2A of the recess 2, occurs. If this force R exceeds the force caused by the abutting contact of the aforesaid planar portions, the main cutting insert 4 is shifted in a rearward direction by an amount of play or clearance between the clamp screw 6 and the main cutting insert 4. As a result, the indexed position of the active cutting edge is changed, so that cutting accuracy is unduly lowered.
In order to circumvent the above problem, a new ball end mill is proposed in Japanese Patent Application, Publication No. 64-87109, in which there is provided a shift preventing means comprising an annular projection 8 formed around the threaded hole in the insert receiving recess 2 and an annular recess 9 on which the annular projection 8 is fitted without any gap formed therebetween as shown in FIGS. 12 and 13.
However, recently the ability to withstand more sever cutting conditions has become a required feature in order to further enhance cutting performance: in the above improved ball end mill, the annular projection 8 may be deformed or worn off since the cutting load is exerted mainly on the projection, which is considerably small in size and height.
Furthermore, Japanese Utility Model Application First Publication, Serial No. 2-31618, describes a shift preventing structure comprising an elongated projection formed on the bottom of the insert and a groove formed in the recess so as to be fitted on the projection.
However, in this end mill, the shape of the insert must be modified, and additionally a high accuracy is required in order to form the elongated projection and groove. Therefore, the manufacturing cost of the insert is unduly increased.
Moreover, in the case where a recess is formed in the bottom of the insert, the strength of the insert is inevitably lowered, and hence the insert becomes susceptible to fracturing during a rough cutting operation. On the other hand, if a projection is formed on the bottom of the insert, the strength of the insert will not be deteriorated. However, it is especially difficult to form the projection at high precision compared with the case of forming the recess. Accordingly, the seating stability of the insert is deteriorated due to the poor accuracy of the processing.